Impedance controlling circuit for a load element



Nov. 24, 1970 H. G. BLANK I 3,543,086

IMPEDANCE CONTROLLING CIRCUIT FOR A LOAD ELEMENT Filed April 19, 1968 lNVENTOR.

HANS G. BLANK ATTO United States Patent 3,543,086 IMPEDANCE CONTROLLING CIRCUIT FOR A LOAD ELEMENT Hans G. Blank, New Rochelle, N.Y., assignor to General Telephone & Electronics Laboratories Incorporated, a

corporation of Delaware Filed Apr. 19, 1968, Ser. No. 722,755 Int. Cl. H05b 37/00, 39/00 US. Cl. 315-169 4 Claims ABSTRACT OF THE DISCLOSURE An impedance-controlling circuit is provided in which the impedance of the primary winding of a transformer is controlled by changing the effective load on the transformer secondary winding. The primary winding is connected in series with an AC voltage source and a load element, so that the amount of voltage appearing across the load element depends on the primary winding impedance. A transistor is used in conjunction with a pair of diodes to effectively open-circuit and short-circuit the secondary winding in response to external control signals applied to the base of the transistor.

BACKGROUND OF THE INVENTION This invention relates to impedance-controlling circuits and in particular to circuits for controlling high imped ance load elements such as electroluminescent devices.

In general, an electroluminescent device comprises an electroluminescent phosphor interposed between two electrically conductive electrodes, at least one of which is transparent. When a varying voltage of appropriate magnitude and frequency is applied across the electrodes, the electroluminescent phosphor is excited and emits light. Groups of electroluminescent devices are often used to visually display information, and it becomes necessary to switch individual electroluminescent devices on and off in response to electrical control signals. These signals are typically DC voltage levels, of the order of a few volts, generated by logic circuitry. However, most electroluminescent devices require voltages of the order of hundreds of volts. At operating frequencies, a typical electroluminescent device has an impedance of the order of tens of thousands of ohms. Therefore, a relatively large AC voltage impressed across a high impedance load must be controlled with a relatively small DC control voltage.

In the prior art there have been many approaches to this problem. The classical method is to employ a relay which engages and completes an AC circuit upon application of a DC signal. More recently, semiconductor devices have been used to effect the desired switching. One method uses a lamp-photoconductor combination. A small control signal is used to energize and deenergize the lamp which is light-coupled to the photoconductor element. The photoconductor element is connected in series with an electroluminescent device and an appropriate AC voltage source. The impedance of the photoconductor element is controlled by the light from the lamp. When the lamp is off, the photoconductor element has a high impedance and only a small portion of the AC source voltage is applied across the electroluminescent device. When the lamp is energized, however, the impedance of the photoconductor element is decreased with a resultant increase in the voltage applied across the electroluminescent device. A basic disadvantage of this method is that lamps are limited-life components and must be replaced periodically.

Another method of controlling the voltage across a high impedance load element is disclosed in US. Pat. 3,371,230 issued Feb. 28, 1968 to H. Blank et al. The

3,543,86 Patented Nov. 24, 1970 circuits disclosed therein utilize four-layer semiconductor devices, such as silicon controlled rectifiers, to achieve the desired impedance control. While these circuits operate in the intended manner, it would be advantageous to have an impedance-controlling circuit which utilizes conventional semiconductor transistors, since they are generally cheaper and easier to obtain than silicon controlled rectifiers.

Accordingly, it is an object of this invention to provide a transistor circuit for the control of high impedance load elements, such as electroluminescent devices.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to an impedancecontrolling circuit in which an externally applied signal is used to control the impedance of an AC circuit element.

According to the present invention, the impedance of the primary winding of a transformer is controlled by changing the effective load on the secondary winding. The transformer primary winding is connected in series with a load element, such as an electroluminescent device, and an AC voltage source. The AC voltage applied across the load is dependent upon the primary winding impedance, since the primary winding and the load divide the voltage supplied by the AC voltage source. Therefore, the voltage applied across the load can be controlled by changing the impedance of the primary winding.

In my invention, first and second unidirectional current means, poled in opposite directions, are connected in series across the transformer secondary winding. A pair of diodes connected back-to-back may be used for this purpose. A semiconductor switching element, such as a transistor having conductive and non-conductive states, couples the junction of the diodes to an intermediate terminal on the secondary winding. An externally applied signal is used to render the transistor conductive and non-conductive.

In operation, the AC signal applied across the primary winding induces voltages in the secondary winding. When the transistor is non-conductive, the oppositely poled diodes prevent current from flowing in the secondary Winding. In this condition, the secondary winding is virtually open-circuited and the primary winding is, therefore, in a high impedance state. (In general, the impedance looking into the primary winding of a transformer will assume its highest value when the transformer secondary is open-circuited, and its lowest value when the secondary is short-circuited.) When the transistor is conductive, voltages induced in the secondary winding cause current flow in part of the secondary since a current path is provided by one of the diodes in conjunction with the transistor. In this condition, part of the secondary wind ing is virtually short-circuited and, therefore, the primary winding is in a low impedance state. The circuit arrangement allows the use of a conventional transistor, since the diodes restrict current flow to a direction compatible with the conductive direction of the transistor regardless of the polarity of the induced secondary voltage.

Further features and advantages of the invention will become more readily apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The drawing is an electrical schematic diagram of a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, a transformer 10 is shown having primary Winding 11 and secondary winding 12. An electroluminescent device 15 and voltage generating source 16 are coupled in series across primary winding 11. The secondary winding 12 of transformer has an intermediate tap connection to a ground reference which divides secondary winding 12 into approximate half-windings. First and second semiconductor diodes 20 and 21, poled in opposite directions, are connected in series across secondary winding 12. Transistor 23, illustrated as of the NPN type, provides a controllable current path between junction point 27 and the intermediate tap of secondary winding 12. Transistor 23 is shown in a common-emitter configuration, having its collector electrode connected to junction point 22, its emitter electrode connected to ground reference, and its base electrode connected to input terminal 14. Input signals applied at terminal 14 are measured with respect to ground reference.

The output of voltage generating source 16 is an AC voltage of the order of 100 volts R.M.S. at 2000 cycles per second. The electroluminescent device requires a substantial AC voltage for excitation, for example 90 volts R.M.S. at 2000 cycles per second, and presents a high load impedance, of the order of 50,000 ohms, at said frequency. In operation, the impedance of primary winding 11 is controlled by the signal at input terminal 14. The voltage applied across the electroluminescent device 15 is thereby controlled since electroluminescent device 15 and primary winding 11 divide the voltage supplied by voltage generating source 16.

Transistor 23 is operated as a switch being either conducting or cut-01f depending on the signal at input terminal 14. When transistor 23 is cut-off or nonconductive, secondary winding 12 is virtually open-circuited because oppositely poled diodes 20 and 21 prevent current flow in said winding. In this condition, the impedance looking into primary winding 11 is essentially determined by the primary inductance of transformer 10. At the operating frequency, this primary inductance presents a high impedance, of the order of 150,000 ohms, to the AC voltage supplied by voltage generating source 16. Therefore, the portion of the supplied voltage which is applied across electroluminescent device 15 is relatively small, of the order of 25 volts R.M.S. At this voltage, the electroluminescent device is off, as it does not emit discernible light.

When transistor 23 is conductive, however, a current path is provided between junction point 27 and the intermediate tap of secondary winding 12. Diodes 20 and 21 conduct alternately as the polarity of the induced voltage changes every half-cycle, and a virtual short circuit exists continually across part of secondary winding 12. In this condition, the impedance looking into primary winding 11 is low, of the order of 2000 ohms, being essentially determined by the winding resistances and leakage inductance of transformer 10. Therefore, the voltage applied across the electroluminescent device is a large portion of the supply voltage, of the order of 96 volts R.M.S. At this voltage, the electroluminescent device is Transistor 23 is switched from cut-off to conduction and vice versa by relatively small DC voltage levels. The difference between the voltages required at terminal 14 to switch between conductive and non-conductive states is of the order of 1 volt.

The diodes 20 and 21 and the transistor 23 are subjected to maximum voltage stresses when transistor 23 is cut-off. These stresses are proportional to the magnitude of the voltages induced in secondary winding 12. By using a stepdown transformer 10, the magnitude of voltages induced in secondary winding 12 can be fixed at a desired level. A stepdown ratio for transformer 10 of five primary turns to one secondary turn is preferred.

This ratio should be at least three to one to avoid unnecessary voltage stresses.

0 In a typical circuit, the values of the components are as follows:

Transistor 23 Type 2N1605. Diodes 20 and 21 Type 1N536. Primary winding 11 200 turns. Secondary winding 12 turns. Electroluminescent device 15 40,000 ohms at 2000 c.p.s. AC voltage source 16 100 volts R.M.S. at 200 c.p.s.

With the above understanding of the invention it will be understood that variations may be made. For example, transistor 23 may be of the PNP type if the polarities of diodes 20 and 21 as Well as the input signal polarity are reversed. Further changes and modifications may be made all within the scope of the appended claims.

I claim:

1. An impedance-controlling circuit comprising:

(a) a transformer having primary and secondary windings, said primary winding having high and low impedance states, said secondary winding having first, second and intermediate terminals;

(b) a load element and a source of AC voltage connected in series across said primary winding;

(c) first and second unidirectional current means connected in series between the first and second terminals of said secondary winding, said first and second unidirectional current means being connected at a junction point, said first and second unidirectional current means being poled in opposite directions,

(d) a semiconductor switching element coupled between the said junction point and the intermediate terminal of said secondary winding, said semiconductor switching element having a conductive and a non-conductive state and being rendered con ductive and non-conductive by an externally applied signal, said primary winding being in its low impedance state when said semiconductor switching element is conductive and being in its high impedance state when said semiconductor switching element is non-conductive.

2. The circuit of claim 1 in which said first and second unidirectional current means are semiconductor diodes and said semiconductor switching element is a transistor.

3. The circuit of claim 2 in which said transformer is a stepdown transformer having a stepdown ratio of at least three primary turns to one secondary turn.

4. The circuit of claim 3 in which said load element is an electroluminescent device, said device being 55 switched on and off by said externally applied signal.

References Cited UNITED STATES PATENTS 3,165,640 1/1965 Fitzwater 307-229 3,280,341 10/1966 Duvall 315-169 3,327,163 6/1967 Blank 3l5-l69 3,409,876 11/1968 Uphoff 315-169 X 3,432,724 3/1969 Frost 315-169 JAMES D. KALLAM, Primary Examiner A. J. JAMES, Assistant Examiner US. Cl. X.R. 370229; 340166 

